Wednesday 7 March 2012 13.11 EST
First published on Wednesday 7 March 2012 13.11 EST

Sheffield is one of the few theatres to devote seasons to living writers. Now it is the turn of Michael Frayn. With Democracy still to come, it is already possible to reassess individual plays, dwell on production methods and delight in Frayn's skill in drawing philosophical meanings out of domestic or historical crises.

Not having seen Benefactors since its 1984 premiere, I had forgotten just how rich a play it is. On one level, it reminds us of the way architectural ideals are compromised by economic realities. We watch, with amused dismay, as the principled David is forced by town planners to turn a south London development project, modelled on collegiate quadrangles, into a high-rise monstrosity. It's a subject of particular relevance in Sheffield where the Parkhill estate became a blot on the landscape; but Frayn also reminds us of the way just about every British city has suffered from brutalist, skyscraping follies.

Frayn's play is even more lethal when it exposes the failure of good intentions on the private level. David and Jane are a kindly, successful couple who can't do enough for their neighbours, the cynical Colin and the disorganised Sheila. What they don't realise is that, in welcoming them into their home, they are sowing the seeds of their own destruction. The result is a devastating attack on the fallibility of liberal altruism that reminded me of Max Frisch's The Fire Raisers, in which a hapless bourgeois shows similar hospitality towards a pair of palpable arsonists. Smoothly directed by Charlotte Gwinner, and impeccably acted by Simon Wilson and Abigail Cruttenden as the genial hosts and Andrew Woodall and Rebecca Lacey as the dependent visitors, this is a major act of reclamation.

Copenhagen, first seen in 1998, is an even greater play in that it deals with a more momentous issue: what took place in 1941 when Werner Heisenberg, the potential father of a German atomic bomb, visited his mentor and former colleague, Niels Bohr, in occupied Denmark. Was Heisenberg seeking information, absolution or endorsement? We are never quite sure. But Frayn uses this historic uncertainty to create a deeply moving meditation on memory, scientific morality and the difficulty of ever knowing either our own or other people's motives.

Part of the play's effect depends on the merging of theme and form, with the actors orbiting the stage like electrons, neutrons and protons. But for that you need a clear, well-lit space whereas, in David Grindley's production, Jonathan Fensom's design thrusts the actors upstage in a quasi-realistic house and the lighting puts faces in shadow at key moments. Fortunately, Henry Goodman as Bohr, Geoffrey Streatfeild as Heisenberg and Barbara Flynn as Bohr's inquisitive wife, Margrethe, deliver Frayn's text with exemplary clarity and remind us of his extraordinary capacity to combine nuclear physics with metaphysical speculation on life's mystery.